In an audio system for vehicles, for reinforcing lower band (buss) sounds, a subwoofer will be additionally installed on a tray or trunk of a vehicle. FIG. 7 shows an example of vehicle audio system how the speakers are arranged in a sedan type vehicle. Four full range speakers S1-S4 are installed on the left and right doors in the front and rear, respectively, and a speaker S5 for lower band reproduction is mounted on a trunk as a subwoofer. In an RV (recreational vehicle) or minivan type vehicle, the lower band reproduction speaker S5 will be established at the back end of the luggage compartment.
Since it is a high power loudspeaker, a separate amplifier is required for driving the subwoofer. In many cases, such a power amplifier for driving the subwoofer includes, in addition to the amplifier unit, functions of a parametric equalizer and a filter for compensating or correcting the sound property unique to the vehicle. Traditionally, settings and adjustment of such functions are conducted with the use of analog electrical components ALP, such as rotary volumes, capacitors, switches, etc., provided at a side surface SD of the power amplifier such as shown in the external view of FIG. 8.
With the recent advancement in the digital signal technology and the cost reduction in the digital components, the settings and adjustment of various correction functions in such vehicle power amplifiers can be fulfilled by using the digital signal technology. With use of the digital technology, the performance and functions can be improved as well as such functions can be achieved at lower cost. However, complications will arise in the coarse adjustment of the vehicle power amplifier prior to the installment in a vehicle, which was easy in the case of using the analog circuit technology.
Namely, when installing such a vehicle power amplifier having sound property compensation functions in a vehicle, the setting made by the manufacturer before the shipment will not be an optimal setting for the vehicle. In a worst case scenario, an extremely high sound volume can be reproduced when the power is turned on due to an unpredictable setting value, which may damage other components or devices as well as harm people inside the vehicle. Therefore, to avoid such a problem, when a power amplifier is sold at a store or the like, an installer has to set and adjust (coarse adjustment) the various compensation functions in the vehicle power amplifier in conjunction with the specific type of vehicle or the particular audio system in the vehicle prior to the installment.
However, due to the digital signal technology, this coarse adjustment becomes difficult because such adjustment is not visually observable. Namely, in the analog circuit, the default values set in the power amplifier can be confirmed by visual inspection prior to the installment. In contrast, when using the digital signal technology, this default values in the amplifier cannot be confirmed unless connected to a power source.
Thus, in one approach, before installing in a vehicle, various functions of the power amplifier can be set and adjusted by connecting the power amplifier to a power source of a small power capacity. However, since the power consumption in the amplifier unit is high, such a power source of small power capacity cannot accommodate the required power level, which in turn may damage the power source.
Japanese Utility Model Publication No. Hei 5-37532 published Sep. 22, 1993, discloses a sound volume adjustment circuit using electronics volume developed for audio equipment. The sound volume adjustment circuit includes a main power source for supplying an electric power to the entire equipment, a sub-power source for supplying an electric power to a sound level controller and a display, and a mode setting means for establishing a standby mode for supplying electric power to the sound level controller and display through the sub-power source.
In this arrangement, a user sets the standby mode and adjusts the sound level of the electronics volume with use of the sound level controller and the display while visually monitoring the sound level. Such change-over to the standby mode is made by operating a remote control switch established on a remote controller or a standby switch established on a receiver. According to this conventional technology, the power consumption during the sound adjustment process can be reduced, thus, allowing to adjust the sound volume by simply connecting the amplifier to a small capacity power source.
Similar to such a conventional technology in the audio equipment noted above, a vehicle power amplifier can also be adjusted their various compensation functions with use of a sound volume adjustment circuit before installing in a vehicle. However, in this conventional technology, the mode can easily be switched to the standby mode or other modes by operating the remote control switch or the standby switch. Thus, in the case when the adjustment circuit is not in the standby mode when the power amplifier is turned on, a large amount of power will be consumed, which may damage the small capacity power source.
In addition, if, the power amplifier is installed in the vehicle without the sound volume adjustment such as by an error, and the power is turned on, an extremely high volume sound may be reproduced because the power amplifier is in unpredictable settings, which in turn may damage the devices as well as harm the people inside the vehicle.